Failing specs for embedded doc position updating

[johnnyshields]

This PR currently demonstrates divergent behavior where the following are not equivalent on embedded docs:

# this correctly preserves order
embedded_doc.move_to(2)
embedded_doc.save!

# this does NOT preserve order and just literally sets the position field
embedded_doc.position = 2
embedded_doc.save!

This behavior does work correctly on non-embedded docs.

(TBD: perhaps we need an option to preserve legacy direct position setting behavior, but i doubt it...)